It is often necessary or desirable to provide an indication, or to take remedial action, when a leak of a predetermined magnitude develops in a hydraulic system. To accomplish this, it is necessary to continuously monitor the condition of the hydraulic system even though portions of such system may be remotely located. Leak detection is particularly important in hydraulic systems used to position the control surfaces of an aircraft. If a leak of a predetermined magnitude develops in one of the hydraulic systems of an aircraft, it is essential that such system be shut off from the source of hydraulic fluid under pressure to prevent complete loss of hydraulic fluid and possible destruction of the aircraft. A hydraulic circuit breaker or hydraulic fuse is utilized to accomplish the leak detection and shut-off functions.
In a typical hydraulic control system, it is necessary to supply hydraulic fluid to, and return hydraulic fluid from, a hydraulic device. The hydraulic device may include, for example, a hydraulic actuator for positioning a member, such as an aircraft control surface and a servovalve for controlling the hydraulic actuator.
In a hydraulic system of this type, the return flow is a function of the supply flow if there are no leaks in the system. Accordingly, leak detection for this type of system can be accomplished by comparing the supply and return flows.
For example, Byford U.S. Pat. No. 3,685,531 shows a hydraulic circuit breaker which compares supply and return flows and shuts off the supply flow if the supply flow exceeds the return flow by a predetermined amount.
With the patented device supply and return flows are measured by a slide valve which is positioned so as to maintain the pressure drops across supply and return orifices substantially constant. A separate spool valve is responsive to the ratio of the pressure drops across the two orifices reaching a predetermined value for closing the supply conduit. Thus the patented device uses two separate slide valves.